The overall organization of urinary acidification has been defined by a variety of studies of the intact kidney and individual nephrons in vivo, yet relatively little is known about the transport processes across the cell membranes, their rate-limiting factors and their organization within the epithelium. The proposed research is concerned with an examination of the active and passive components of transport in acidification by the isolated urinary bladder of the water turtle. The role of the different components and the factors controlling them will be defined. Current studies indicate that the rate of active H ion transport is controlled by the H ion gradient across the luminal membrane, by the quantities of CO2 that can be hydroxylated within the epithelium and by as yet poorly defined energetic factors. The net rate of acidification is determined also by bicarbonate flows. During acidification against a gradient there is a net flow of bicarbonate down an electrochemical gradient. In the isolated turtle bladder this bicarbonate flow is coupled to chloride absorption and is of sufficient magnitude to be a major determinant of the net rate of acidification. Current studies appear to indicate that this transport system depends on metabolic energy and is located at the luminal membrane of the epithelium. The characteristics of this mechanism for bicarbonate secretion will be further defined and its role in the regulation of acidification will be explored. These studies on the functional organization of the transport processes will be correlated with morphological studies on the fine structure of the turtle bladder.